FIG. 1 depicts a schematic diagram of an exemplary wireless local-area network (LAN) 100 in the prior art comprising access point 101, stations 102-1 through 102-M, wherein M is a positive integer, and shared communications channel 103, interconnected as shown. Each station 102-i, wherein i is a member of the set {1, 2, . . . M}, is a device such as a notebook computer, personal digital assistant (PDA), tablet PC, etc. that transmits radio signals to and receives radio signals from other stations in local-area network 100 via access point 101.
Access point 101 and stations 102-1 through 102-M transmit data blocks called frames over shared-communications channel 103 such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more frames can become corrupted (resulting in a collision). Local-area networks therefore typically employ a Medium Access Control (MAC) protocol for ensuring that a station can gain exclusive access to shared-communications channel 103 for an interval of time in order to transmit one or more frames.
In wireless local-area networks based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, the MAC protocol is based on a Carrier Sense Multiple Access (CSMA) mechanism in which a station or access point can detect whether shared-communications channel 103 is busy or idle. If shared-communications channel 103 is busy, a station or access point will wait until the channel is idle before attempting to transmit a signal.
A common technique for achieving increased data rates in telecommunications networks is to use more spectrum for transmitting and receiving signals. Accordingly, stations and access points could communicate with higher data rates in wireless local-area networks by transmitting and receiving higher-bandwidth (i.e., “wide”) signals over a combination of two or more shared-communications channels. Such stations and access points are referred to subsequently in this specification as enhanced stations and enhanced access points, respectively.
A major obstacle in deploying enhanced stations and access points, however, is the huge installed base of legacy stations. In particular, the new, wider waveforms of enhanced stations might not be recognized by legacy stations. Consequently, a legacy station might incorrectly identify a shared-communications channel as idle and transmit a signal over the channel, resulting in a collision. There is therefore a need for enhanced stations and access points that can coexist with legacy stations in local-area networks without any deleterious effects on the operation of the legacy stations, or on the operation of the enhanced stations. (Since a legacy access point cannot handle the wider signals of an enhanced station, coexistence between an enhanced station and a legacy access point would require the enhanced station to operate in a “legacy mode.”)